The present invention relates to a method of growing thin films on substrates in a reaction space being subjected to alternately repeated steps of feeding a precursor followed by feeding a catalyst for the precursor to grow the desired thin film.
There are a variety of methods employed in the art of growing thin films. For example, chemical vapor deposition (CVD) has been used for many years to deposit solid thin films for a variety of commercial products including integrated circuits. The CVD method consists of exposing a substrate in a reaction chamber to one or more gaseous species that react to form a solid thin film on the substrate surface. CVD may be carried out at atmospheric or sub-atmospheric pressures, and usually requires that the reactivity of the gases be enhanced by heating the substrate or by creating a glow discharge in the reaction chamber. One of the disadvantages of the CVD method is that the gases may take part in undesired gas phase reactions. These undesired gas phase reactions may lead to the formation of particulates, and may also deplete the seed gases before they can reach all areas of the substrate. Thus, the undesired gas phase reactions can impact the quality and uniformity of the thin film being deposited.
An example of a specific CVD process is disclosed in U.S. Pat. No. 6,110,530 assigned to the assignee of the present application. The ""530 patent discloses a CVD process for depositing a solid thin film of copper that employs a single gaseous precursor to form the copper layer. Although that precursor, hexafluoracetylacetonate-Cu-trimethylvinylsilane (hereinafter hfac-Cu-TMVS), is sufficiently reactive to deposit copper at temperatures below 200xc2x0 C., the copper deposition rate is undesirably slow. It is, however, undesirable to raise the processing temperature to increase the reaction rate because, among other reasons, higher processing temperatures may degrade previously deposited layers on the substrate. It was found that the slow deposition rate at lower temperatures could be increased by adding water to the hfac-Cu-TMVS precursor. It appears that the water may act as a catalyst, facilitating the decomposition of hfac-Cu-TMVS at lower temperatures. Although the addition of water avoids the need to carry out the CVD deposition at higher temperatures, the problem of undesired gas phase reactions remains. Specifically, the addition of water also facilitates the decomposition of hfac-Cu-TMVS in the gas phase, which can lead to problems such as particulate formation. In other words, catalytically converting the copper precursor to elemental copper with water during a single processing step within the reaction chamber, although having certain advantages, suffers from processing limitations to which the present invention is intended to address.
When a CVD reaction requires more than one gaseous reactant, undesired gas phase reactions between them can by avoided by separately and sequentially introducing the gaseous reactants into the reaction chamber. One method that employs the separate and sequential introduction of gaseous reactants is the Atomic Layer Deposition (ALD) method, one version of which is described in U.S. Pat. No. 4,058,430. In ALD, the substrate is heated to a temperature such that when a first gas is introduced into a reaction chamber, it chemisorbs on the substrate surface, forming a monolayer. An exact monolayer can be formed because the first layer of the gaseous species is relatively strongly bonded to the surface of the substrate by the chemisorption reaction while any excess reactant is relatively weakly bonded to the chemisorbed monolayer. The excess first gaseous reactant can then be removed from the reaction chamber. This removal may take place by, for example, evacuating the reaction chamber with a vacuum pump or by purging the reaction chamber with an inert gas. Ideally, the monolayer of the first gaseous species remains on the heated substrate surface after the removal of the excess reactant. Next, a second gaseous reactant is introduced into the reaction chamber. The second gaseous reactant reacts with the monolayer to produce the desired solid thin film. The excess of the second reactant, along with any reaction by-products, are then removed from the reaction chamber. Again, the removal may take place by means of evacuating the reaction chamber or purging the reaction chamber with an inert gas. A distinguishing characteristic of the ALD process is that it deposits a precise layer thickness each time the above sequence of steps is repeated. A precise layer thickness is obtained because of the formation of an exact monolayer of the first precursor. The above sequence of steps can be repeated until a thin film of desired thickness is created.
Although ALD may eliminate undesired gas phase reactions between multiple CVD precursors, ALD has a number of disadvantages that limit its utility. One of those disadvantages is that the ALD process can only be used with gaseous species that react with the substrate and with each other within a xe2x80x9ctemperature windowxe2x80x9d. The temperature window is defined by the constraints that the substrate temperature must be high enough for the second gaseous reactant to react with the chemisorbed monolayer of the first gaseous reactant, but the substrate temperature cannot be so high that the monolayer of the first reactant desorbs. This temperature window severely limits the number of species that are compatible with the ALD process. Furthermore, the temperature window limitation of ALD means that ALD cannot be used to deposit a film from a single gaseous species. The use of a single species is incompatible with ALD because the substrate temperature would have to be high enough for the species to react to form a solid film. Since the film formation reaction would consume any of the species introduced into the reactor, the precise thickness control characteristic of ALD would not be possible. In other words, the deposition process would degrade into CVD with a single reactant. Thus the precise layer control obtainable in ALD, which is made possible by separating the introduction of the gaseous reactants, cannot be achieved in a single species system.
The present invention is directed to a method of growing a thin film onto a substrate. The method comprises feeding a single gaseous precursor into a reaction space. The precursor is caused to adsorb onto the surface of the substrate to form a layer. A catalyst is then fed into the reaction space in an amount to substantially convert the layer of the precursor into the desired thin film. Embodiments of the present invention overcome the shortcomings of previously available CVD and ALD methods.